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How to design a passive house for summertime comfort

In the context of climate warming, the topic of resistance to overheating is gaining more and more importance, especially since high indoor temperatures are not only a matter of comfort, but also largely related to health. Summer temperatures inside a building are greatly influenced by solar loads, internal heat sources and ventilation strategy.

Optimizing the design to the local climate conditions has a fundamental influence on the

  • summer thermal comfort achieved and

  • robustness of the building to the risk of overheating in adverse circumstances.

Proof of high comfort throughout the year is at the heart of the passive house concept. The perception and measurement of thermal comfort is a complex subject in itself, as it is influenced by many aspects, e.g. temperature and humidity, air movement, clothing types, activity and even personal preferences. A significant indicator for summer comfort - which has been proven in practice - is the temperature, which should not exceed the comfort level for long periods of time. As part of quality assurance measures to ensure high thermal comfort in summer, the frequency of overheating is limited for Passive House buildings. More specifically, the indoor temperatures of a passive house building should not exceed 25 °C for more than 10% of the hours in a year and it is recommended to be below 5%.

A risk analysis through design stress testing is absolutely essential to a robust summer comfort strategy. If the risk of overheating cannot be reliably limited, active cooling is essential to maintain high summer comfort. In this case, the energy demand for active cooling must be kept below a certain threshold.

This guide to summer comfort has been written to raise awareness and assist building designers in developing a robust strategy to ensure year-round thermal comfort. The design aid provided helps identify effective passive cooling techniques and analyze potential summer comfort risks of a project.

Buildings that can be kept comfortable relying only on passive cooling in today's climate may need active cooling by mid-century. Ideally, a possible future cooling upgrade should be taken into account during design to ensure that the building is designed for a low cooling load. Finally, it is important to provide residents with relevant information and operating instructions, e.g. in the form of a user manual for summer comfort.

Passive cooling measures

The temperature rise inside a building is determined by net heat gains. The first principle of passive cooling is therefore to reduce any potential heat source, e.g. solar gains and heat generated inside the building. When temperatures rise above comfort levels, the only effective way to passively remove excess heat is through ventilation, at times when outside temperatures are sufficiently low.

Heat sources in the building

Heat sources inside the building affect the internal load in summer and should be kept as low as possible. Therefore, the following two aspects should be designed carefully.

  • A domestic hot water (DHW) system with low heat loss, e.g. short pipes, good insulation, low distribution temperature.

  • DHW pipes should be short and well insulated (including valves and fittings) to reduce heat loss. A "good thermal insulation standard" means an insulation thickness of at least twice the nominal pipe diameter (2 x DN)

  • Central DHW systems usually have higher heat losses due to the long distribution network and water circulation. Low distribution temperatures and demand-based control strategies can help reduce losses. Technical solutions must be hygienically problem-free, e.g. ultrafiltration, thermal interface unit in each apartment or chlorination.

  • Storage tanks must be sufficiently insulated and operate at low temperatures.

Energy efficient equipment and appliances

The internal thermal loads are determined by the building occupants themselves, as well as by any technical equipment in the building, e.g. lighting, household or office appliances and auxiliary electricity. During the design of the building, these parameters must be carefully considered in the energy balance. In addition, internal heat loads should be kept low by installing energy-efficient lighting and equipment.

Insulation level

In general, improved insulation reduces heat exchange through a surface. Thus, it also protects a cold interior volume from hot surroundings, as is the case with a refrigerator. If outside temperatures are higher than inside the building, transmission loads through building components (eg. wall, roof or window) are reduced when built to the improved level of passive house insulation. The improved insulation also means that potentially beneficial transmission losses are reduced during summer periods when it is cooler outside than inside (e.g. at night).

In such periods, however, ventilation is in any case a much more effective way of reducing the internal temperature. Together with a suitable ventilation and shading concept, the level of insulation helps to keep indoor conditions cool. It can also be considered a resilience measure against warming climate conditions, as improved insulation is an important measure to keep active cooling demand low.

Night ventilation

Increasing the ventilation rate at times when it is colder outside than inside helps passively cool the building. For example, at night the cold air can be used to dissipate the excess heat accumulated during the day. There are two main strategies: additional window ventilation or additional mechanical ventilation (bypassing heat recovery) at times when outside temperature and humidity are lower than inside (ie especially at night and early morning).

Night ventilation is most effective if applied over a longer period of time, e.g. throughout the night instead of just the evening or morning hours. Short periods of ventilation cool the air and improve the immediate temperature sensation, but the heat stored in the building materials (thermal mass) can only be dissipated with longer periods of ventilation.

A high window air exchange rate could indicate that the cooling strategy is too dependent on the airflow from opening the windows at night or during the day, which in turn is subject to occupant behavior. Practical reasons (e.g. noise or pollution constraints) or personal preferences may lead occupants to implement lower ventilation, which imposes a risk on the building's summer performance. For this reason, a risk analysis is very important to identify how sensitive summer comfort is to user behavior and to guide towards more robust and resilient design choices.

Additional measures for high summer comfort

Additional measures are available to help increase occupant thermal comfort in summer or to reduce cooling loads in case of active cooling. Some examples are described in the next section. To what extent a certain aspect can be exploited for a specific project must be assessed in the context of the building and its use.

  • Increased summer ventilation: a basic ventilation rate is required in each building for reasons of hygiene, which can be provided by either mechanical ventilation, extract air, window ventilation or a combination of the three. If the average outdoor conditions are cooler than the comfort threshold of 25 °C, a higher summer base ventilation rate may be beneficial to keep the building cool. The effectiveness of a higher ventilation rate and its impact on comfort depends on the local climate, i.e. temperature and humidity conditions.

  • Heat recovery bypass: The mechanical ventilation system must have a bypass function. Whenever the outdoor temperature is lower than the indoor conditions, heat recovery should be bypassed so that cold air can enter the building directly. Many ventilation systems today include an automatic bypass function that is controlled based on the outside temperature. If the outdoor temperature exceeds the indoor temperature, mechanical ventilation heat recovery can help pre-cool the incoming supply air.

  • Thermal mass: Thermal mass adds inertia to the building, meaning the building will heat up and cool down more slowly. Absorption of solar or internal loads in the thermal mass of the building has the effect of reducing temperature peaks during sunny periods or periods of high occupancy. To maintain comfortable indoor conditions, it is essential to effectively remove this absorbed excess heat. The only passive means of "recharging" the thermal mass is through nocturnal ventilation for longer periods of time (all night). Therefore, the thermal mass should be optimized for the specific designs considering the load profile and night ventilation potential.

  • Surface colors: dark surfaces have a higher solar absorption than light colored surfaces and therefore heat up noticeably more when exposed to solar radiation. The use of low absorption surface coatings is a simple and effective passive cooling measure. A beneficial side effect of using light colors is the positive influence on reducing the urban heat island effect. In addition to the visible color of the surface, coatings with so-called "cool colors" contribute to keeping surfaces cool due to their high reflectivity in the infrared spectrum.

  • Plants / trees / greenery: Trees or other plants can be aesthetically integrated into the shading concept for summer comfort. In addition, the evaporative effects of plants can have a positive influence on keeping the microclimate around buildings cool. Vegetation is a recommended measure to reduce the urban heat island effect.

  • Fresh air intake pre-cooling: Various techniques of varying complexity are available to pre-cool the intake air of mechanical ventilation systems, e.g. basement heat exchangers (directing fresh air ducts underground before entering the building).

  • Climate-appropriate design: The layout, zoning, orientation and position of a building's windows fundamentally influence summer comfort achievable through passive means. Design considerations that are well established and have a long tradition in hot and warm climate regions can provide cost-effective and reliable methods of keeping a building cool. Examples are architecture with balconies and overhangs to provide shading, shaded courtyards, or design optimized for stack effect and efficient cross ventilation.

Active cooling

If during the design process it becomes clear that the building (or individual rooms) cannot be kept comfortably cool using only passive cooling techniques, active cooling comes into play. In this case, it is important to use passive cooling measures and design the building holistically with the aim of keeping the cooling demand low. Depending on the building and its use, even a small amount of tempering (eg. through supply air) can help reduce discomfort and health problems.

Various technical solutions are available for active cooling – from localized air conditioning to large-scale cooling solutions. A building services engineer should be consulted to design a suitable and efficient system.

Especially in light of climate warming, it makes more and more sense to anticipate active cooling, even if the assessment indicates that comfortable summer conditions can be ensured under current climate conditions. The times when active cooling is required usually align well with the availability of renewable solar energy, meaning that this additional energy demand can be met from sustainable resources. A prerequisite for this is energy efficiency, i.e. ensuring that the building is optimized and designed to keep active cooling demand low. Exploiting the passive cooling techniques and stress testing methodologies described in this guide are essential to energy efficient construction.

Managing indoor temperatures in summer is a matter of comfort and health and must be considered in each individual project. Understanding the building and its likely use, stress-testing the design and assumptions, and adjusting the main influencing factors accordingly are fundamental to achieving an overheating-resistant design.

Author of the guide: Passivhaus Institut ( 2022), Summer confort guidelines in passive house buildings,

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